A study conducted by an international team of scientists has highlighted the challenges of reaching herd immunity against the Delta variant of SARS-CoV-2. Taking Delhi as a case study, the research found that the Delta variant was able to infect people who had previously been infected by SARS-CoV-2. The study, published in the journal Science on Thursday, also found that prior infection gave only 50-90 per cent of the protection against infection with the Delta variant that it provides against previous lineages.

The study also concluded that the Delta variant, which is said to have driven the 2nd wave of the pandemic in India, was 30-70 per cent more transmissible than previous strains of Covid-19 in Delhi.

Delhi witnessed multiple Covid-19 outbreaks, in June, September and November 2020. However, the April 2021 outbreak, when the cases peaked at 20,000, was the worst as it saw a rapid rise in hospitalisations and ICU admissions with the medical infrastructure on the verge of collapse. The daily deaths saw a three-fold rise in comparison with previous waves.

The study noted that Delhi’s seropositivity, which was at 56.1 per cent, should have offered some protection against the Delta variant of Covid-19. But this remained insufficient in providing the protection expected due to the presence of the delta variant. 

The delta variant made it difficult to achieve the herd immunity, which works as an indirect protection from a disease when a sufficient percentage of a population has become immune to an infection.

“The concept of herd immunity is critical in ending outbreaks, but the situation in Delhi shows that infection with previous coronavirus variants will be insufficient for reaching herd immunity against Delta,” PTI quoted study co-author Professor Ravi Gupta from the University of Cambridge as saying.

“The only way of ending or preventing outbreaks of Delta is either by infection with this variant or by using vaccine boosters that raise antibody levels high enough to overcome Delta’s ability to evade neutralisation,” Gupta added.

The work was led by the National Centre of Disease Control and the Council of Scientific and Industrial Research-Institute of Genomics and Integrative Biology (CSIR-IGIB) in New Delhi with collaborators from the University of Cambridge and Imperial College London, UK, and the University of Copenhagen, Denmark.

(With agency inputs)